It is common to use one or more stabilizers in a drill string to keep the string centered and thereby control the inclination of the hole as the bit drills into the earth. A typical stabilizer includes a tubular housing having radially extending blades that is threaded into the pipe. The outer faces of the blades engage the wall of the bore to center the drill string. Where a pair of properly spaced, full-gage stabilizers is used and one is located near the bit, drilling generally will proceed straight ahead. If a near-bit stabilizer is not used and the bore is inclined with respect to vertical, the bit will tend to drill along a path that curves downward due the pendulum effect of the weight of that length of drill pipe which extends downward beyond the uphole stabilizer. If an undergage stabilizer is used uphole in combination with a full-gage stabilizer near the bit, the sag in the drill string at the uphole stabilizer tends to cause the bit to drill along a path that curves upward. Thus to some extent the use and axial positioning of stabilizers can be employed to control the inclination of the borehold in directionally drilled wells.
Another way to change the inclination of a borehole is to use a so-called "bent sub" that can be positioned in the string, for example, above a downhole drilling motor or between the motor and the bearing assembly just above the bit. The conventional bent sub is a length of pipe which has a lower portion formed at an angle to the upper portion thereof. With the sub providing a bend in the pipe, the bit will tend to drill along a path that curves in a plane which contains the two sides or axes of the bent angle, below the bend point. The bit can be steered to some extent to the right or to the left by orienting the plane of the bend with respect to vertical by manipulation of the drill pipe at the surface. Straight-ahead drilling can be resumed by superimposing drill pipe rotation over the rotation of the motor. Although the drill bit will wobble as the bend point orbits about the axis of the borehole, the overall tendency of the bit is to drill a straight hole. Precise control over the borehole inclination can be achieved only where a near-bit stabilizer is used to keep the bit from wandering as it drills, for example, through a dipped bedding plane between two rock strata having different characteristics.
However, the use of a typical stabilizer near the bit impedes the establishment of a bend angle as described above because it resists tilting of the rotation axis of the bit. The blades of the stabilizer engage the wall of the hole for a considerable length that is full gage, and of course the rock resists any tilting of the assembly. This can reduce the effectiveness of using a bend angle to change the course of the borehole in a predictable manner. Yet a near-bit stabilizer is considered to be essential for optimum directional control.
An uphole stabilizer that has been proposed for directional drilling is disclosed in Anderson U.S. Pat. No. 4,848,490 issued Jul. 18, 1989. This device uses spiral blades that carry buttons which can be extended from a minimum to a maximum diameter in response to downward movement of a mandrel within a housing that forms the blades. A spring loaded mechanical detent is used to prevent downward relative movement until a predetermined axial compressive load is applied. However this device is not designed for use as a near-bit motor stabilizer, but rather as an uphole stabilizer which centers the drill string when the buttons are extended, and which allows the string to sag when the buttons are retracted. As disclosed, the stabilizer of the '490 patent does not have many of the features of the present invention. For example, control over the stabilizer requires the application of a certain level of axial compressive force, which can be inadvertently applied during normal drilling operations, or which may not reach the stabilizer at all in a highly deviated well due to wall friction on the pipe. Moveover, a mechanical detent necessarily involves high friction forces, so that tripping can occur at unpredictable levels, particularly as inevitable wear takes place. Rotation of the housing relative to the mandrel cannot occur, so that the stabilizer can not automatically resume its maximum diameter position when the drill string is rotated. Other distinctions also will becomes apparent.
Other problems also occur in providing near-bit stabilization that are not appreciated by the above-mentioned patent. For example, during sliding drilling, the lower portion of the drill string including the motor housing can undergo torsional oscillations as the drill string winds up and unwinds due to variations in weight-on-bit, changes in formation characteristics, strengths of the rocks, bit wear, type of bit, and other variables. As used herein, the term "sliding" drilling means drilling a borehole using only a downhole motor. The drill string is not turned during this type of drilling, but simply slides downward as the borehole is deepened by the bit. Such torsional oscillations can reduce the effectiveness of a variable diameter near bit stabilizer unless precautions are taken to ensure that during sliding drilling the stabilizer remains in its undergage condition even in the presence of such oscillations.
An object of this invention is to provide a new and improved near-bit stabilizer that automatically assumes an undergage condition when a bend angle is being used to directinally drill a borehole.
Another object of the present invention is to provide a new and improved stabilizer that can be operated downhole in a manner such that normally retracted, laterally shiftable members are extended to a full gage diameter in response to rotation of the pipe string.
Yet another object of the present invention is to provide a new and improved downhole adjustable stabilizer having wall engaging means that extend to the full gage of the hole in one mode of operation, and which retract to a lesser diameter when a bend angle is present in the drill string above the stabilizer to enable the rotation axis of the bit to tilt.
Still another object of the present invention is to provide a new and improved adjustable near-bit stabilizer that will remain undergage during sliding drilling in the presence of drill string torsional oscillations.